The present invention relates to high temperature superconducting thick films on polycrystalline substrates with high JC""s and IC""S and to structural template articles for subsequent deposition of an oriented film, e.g., of superconducting thick films.
One process in the production of coated conductors (superconductive tapes or films) has been referred to as a thick film process. In the deposition of thick films for such coated conductors where the thickness of the superconductive layer is generally at least one micron in thickness, the use of polycrystalline substrates, e.g., polycrystalline metal substrates has been preferred. Buffer layers play an important role in the production of high critical current density superconducting films on polycrystalline metal substrates. Suitable buffer layers can provide the necessary structural template for subsequently deposited superconducting layers. For example, a yttria-stabilized zirconia (YSZ) buffer layer deposited by ion beam assisted deposition (IBAD) has been described by both Iijima et al., U.S. Pat. No. 5,650,378, and Russo et al., U.S. Pat. No. 5,432,151. Similarly, Arendt et al., U.S. Pat. No. 5,872,080 described a coated conductor having the structure YBCO/Y2O3/YSZ/Al2O3/Ni alloy with a high critical current density (Jc) of about 1xc3x97106 A/cm2 and a high transport critical current (Ic) of from about 100 to about 200 A/cm. While this current was satisfactory, the deposition of the YSZ layer was considered too slow for commercial production.
In U.S. Pat. No. 6,190,752 by Do et al., thin films of a material having a rock salt-like structure were deposited by IBAD upon amorphous substrate surfaces. Among the preferred materials with a rock salt-like structure was magnesium oxide (MgO). In comparison to the deposition of YSZ, MgO can be rapidly deposited (about 100 times faster) through an IBAD process. The structures of U.S. Pat. No. 6,190,752 included, e.g., YBCO/Y2O3/YSZ/MgO/MgO(IBAD)/Si3N4Ni alloy with a NiO layer in between the YSZ layer and the MgO layer in most instances. Despite the improvement in processing speeds, the structures of U.S. Pat. No. 6,190,752 had Icxe2x80x2s of only about 50 to about 75 A/cm. In addition, at the elevated processing temperatures needed to form the superconductive layer, the silicon nitride layer reacts with other materials in the system.
In U.S. application Ser. No. 09/731,534 by Arendt et al., filed on Dec. 6, 2000, for xe2x80x9cHigh Temperature Superconducting Thick Filmsxe2x80x9d, substrate structures were described including a layer of an inert oxide material such as aluminum oxide on the surface of the polycrystalline metallic substrate, a layer of an amorphous oxide or oxynitride material such as yttrium oxide or aluminum oxynitride on the inert oxide material layer, and, a layer of an oriented cubic oxide material having a rock-salt-like structure such as magnesium oxide upon the amorphous oxide or oxynitride material layer. One exemplary structure described in that patent application included, e.g., YBCO/CeO2/YSZ/MgO(IBAD)/Y2O3/Al2O3/Ni alloy. The critical current density (Jc) was measured as 1.4xc3x97106 A/cm2 using a standard four-point measurement. The projected transport critical current (Ic) was 210 Amperes across a sample 1 cm wide.
The metal oxide, SrRuO3, shows high chemical and thermal stability, and reasonably low electrical resistivity. Due to these properties, SrRuO3 has found applications in various fields. For example, SrRuO3 has been used as a bottom electrode for capacitors where ferroelectric or high dielectric constant perovskite oxides are used as dielectrics, taking advantage of the relatively low resistivity and the compatible structure of SrRuO3 with the dielectric material (see, Eom et al., Appl. Phys. Lett., v. 63, pp. 2570-2572 (1993) and Jia et al., Appl. Phys. Lett., v. 66, pp. 2197-2199 (1995)). Also important was that the interface between SrRuO3 and the ferroelectric materials is chemically stable since all these materials are oxides.
SrRuO3 has also been used in superconductor applications. For example, SrRuO3 combined with platinum (Pt), can be used as a bilayer buffer to grow highly oriented superconducting YBCO on single crystal MgO substrates (Tiwari et al., Appl. Phys. Lett., v. 64, pp. 634-636 (1994)). High temperature superconductor Josephson junctions have also been fabricated using SrRuO3 as a normal metal layer based on an edge-geometry superconductor/normal metal/superconductor configuration (Antognazza et al., Appl. Phys Lett., v. 63, pp. 1005-1007 (1993)).
SrRuO3 and SrRuO3/LaNiO3 have been used as a buffer layer for depositing YBCO on single crystal LaAlO3 substrates (see, Aytug et al., Appl. Phys. Lett., v. 76, pp. 760762 (2000)). Similarly, SrRuO3 and SrRuO3/LaNiO3 have been used as a buffer layer for depositing YBCO on rolling-assisted biaxially textured (RABiTS) substrates (see, Aytug et al., J. Mater. Res., v. 16, no. 9, pp. 2661-2669 (2001)). The conductive oxide strontium ruthenate was specifically described as providing an electrical couple of the high temperature superconductor layer to the underlying metal substrate.
Despite the prior results of Do et al. and Arendt et al., continued improvements in the structure and resultant properties of coated conductors have been desired. After extensive and careful investigation, improvements have now been found in the preparation of superconducting films on polycrystalline substrates such as flexible polycrystalline metal substrates. In particular, strontium ruthenate (SrRuO3) has now been used as a buffer layer directly on an IBAD-deposited magnesium oxide layer.
It is an object of the present invention to provide superconducting films, especially YBCO superconducting films, on polycrystalline substrates such resultant articles demonstrating properties such as high Jc""s and Ic""s.
It is another object of the present invention to provide structural template articles for subsequent deposition of oriented films, e.g., superconducting films, especially YBCO superconducting films.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides an article including a substrate, a layer of an inert oxide material upon the surface of the substrate, a layer of an amorphous oxide or oxynitride material upon the inert oxide material layer, a layer of an oriented cubic oxide material having a rock-salt-like structure upon the amorphous oxide or oxynitride material layer, a layer of SrRuO3 as a buffer material upon the oriented cubic oxide material layer. In a preferred embodiment, the article is a superconductive article and further includes a top-layer of a HTS material directly upon the SrRuO3 buffer layer.
In another embodiment of the invention, the present invention provides an article including a substrate, a layer of an amorphous oxide or oxynitride material upon the substrate, a layer of an oriented cubic oxide material having a rock-salt-like structure upon the amorphous oxide or oxynitride material layer, and a layer of SrRuO3 buffer material upon the oriented cubic oxide material layer. In another preferred embodiment, the article is a superconductive article and further includes a top-layer of a HTS material directly upon the SrRuO3 buffer material layer.
The present invention also provides a thin film template structure including a flexible polycrystalline metal substrate, a layer of an inert oxide material upon the surface of the flexible polycrystalline metal substrate, a layer of an amorphous oxide or oxynitride material upon the inert oxide material layer, a layer of an oriented cubic oxide material having a rock-salt-like structure upon the amorphous oxide or oxynitride material layer, and a layer of SrRuO3 as a buffer layer on the layer of an oriented cubic oxide material. The thin film template structures of the present invention are useful for subsequent epitaxial growth of perovskite oxide thin films.